During the past several years, sequential lateral solidification (“SLS”) techniques have been developed to generate quality large grained polycrystalline thin films, e.g., silicon films, having a substantially uniform grain structure. For example, in U.S. Pat. No. 6,322,625, issued to Im and U.S. patent application Ser. No. 09/390,537 (the “'537 application”), the entire disclosures of which are incorporated herein by reference, particularly advantageous apparatus and methods for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses and small-scale translation of a silicon sample to implement sequential lateral solidification have been described. As described in these patent documents, at least portions of the semiconductor film on a substrate are irradiated with a suitable radiation pulse to completely melt such portions of the film throughout their thickness.
In order to increase throughput, continuous motion SLS processes have been proposed. Referring to FIG. 1, such system preferably includes an excimer laser 110, an energy density modulator 120 to rapidly change the energy density of a laser beam 111, a beam attenuator and shutter 130, optics 140, 141, 142 and 143, a beam homogenizer 144, a lens and beam steering system 145, 148, a masking system 150, another lens and beam steering system 161, 162, 163, an incident laser pulse 164, a thin film sample on a substrate 170 (e.g., a silicon thin film) a sample translation stage 180, a granite block 190, a support system 191, 192, 193, 194, 195, 196, and a computer 100 which manages X and Y direction translations and microtranslations of the film sample and substrate 170. The computer 100 directs such translations and/or microtranslations by either a movement of a mask within masking system 150 or by a movement of the sample translation stage 180. As described in U.S. Pat. No. 6,555,449 issued to Im, the entire disclosure of which is incorporated herein by reference, the sample 170 may be translated with respect to the laser beam 149, either by moving the masking system 150 or the sample translation stage 180, in order to grow crystal regions in the sample 170.
FIG. 2 depicts the mask used in the continuous motion SLS process as described in International Publication No. 02/086954 (the “'954 Publication”), the entire disclosure of which is incorporated herein by reference. This mask is divided into a first mask section 20 and a second mask section 22. The first mask section 20 can be used for the first pass under the laser. The second mask section 22 is used on the second pass. The first mask section 20 may have corresponding opaque areas 24 and clear areas 25. Throughout the specification of the '954 Publication and the present application, “opaque areas” are referred to as areas of the mask that prevent associated regions of a thin film sample irradiated by beams passed through the mask from being completely melted throughout its thickness, while “clear areas” are areas of the mask that permit associated regions of a thin film sample irradiated by beams passed through the mask to be completely melted throughout its thickness. The clear areas can be actual holes in the mask or may be sections of the mask that allow the sample behind it to be completely melted throughout its thickness. The second mask section 22 also has corresponding opaque areas 26 and clear areas 27. The opaque areas 24, 26 of both sections 20, 22 are areas that prevent radiation from a laser source from passing through to the sample. The shape of these clear areas, both in the second mask section 22 and in the first mask section 20, generally have a shape of “straight slits.” The array of the clear areas 24 in the first mask section 20 are generally staggered from the array of clear areas 26 in the second mask section 22. As indicated above, the clear areas 25, 27 of both sections allows radiation to pass through to melt the sample below the surface of the mask.
FIG. 3 depicts the radiation pattern passing through the mask of FIG. 2 during processing of the film. The first pattern section 30 shows the pattern that results after the first pass of the irradiation by the pulses shaped using the mask. The pulse passing through the mask may have a first portion 34 that corresponds to the pattern of the first mask section 20. The clear areas of the first mask section 20 in FIG. 2 allow the radiation to pass therethrough, and melt the thin film throughout its thickness, thus resulting in a first melted region and an unmelted region 44 (see FIG. 4) after the first pass of the sample processing. When the mask is translated in the direction of the arrow 33, the second pattern section 32 of FIG. 3 with the radiation pattern results after the second pass of processing the sample. The pulse passing through the mask may have a second portion 36 that corresponds to the pattern of the second mask section 22. The clear areas of the second mask section 22 of the mask in FIG. 2 allow the radiation to pass therethrough, and again melt the thin film throughout its thickness. This results in a second melted region and an unmelted region over the grain boundary 45 (see FIG. 4).
FIG. 4 depicts the resulting crystalline structure that is produced using the mask of FIG. 2. The first structure section 40 includes the structure 41 that results after the first pass of the sample processing. The opaque areas of the first mask section 20 of the mask of FIG. 2 prevent the associated regions 44 from completely melting. A grain boundary 45 in the direction of the crystalline structure forms approximately halfway between the associated regions 44. The second structure section 42 includes the crystalline structure 48 that results after the second pass of the sample processing. The grain boundary 45 from the first pass is not removed, while the individual grains expand in length until they meet one another, because all areas are exposed to the laser during the second pass except the area that corresponds to the grain boundary 45. Thus, the grain length 46 (parallel to the direction of the crystalline structure) may be controlled by the properties and slit patterns of the mask of FIG. 2. The width 47 of the grain (perpendicular to the direction of the crystalline structure), however, is not very easily controlled. Indeed, it may be primarily dependent on the characteristics of the film.
As noted above, the aforementioned SLS techniques typically employ a straight slit mask pattern. This allows for the ease of control of the grain length (in the direction of the primary crystallization). In such case, the perpendicular grain spacing may be dependent on the properties of the film, and thus is not very easily manipulated. While the tailoring of the shaped areas to manipulate the microstructure has been employed in other SLS methods and systems, such as with the use of chevron-shaped openings in a mask, the techniques associated therewith may produce narrow grain areas. Accordingly, there is a need to control grain length in the thin film, as well as increase the area in which a smaller number of grains are present.